To date, for example, a technology described in Japanese Unexamined Patent Application Publication No. 2006-116552 is known as a method of manufacturing a part having a closed cross section.
Japanese Unexamined Patent Application Publication No. 2006-116552, the following steps are successively performed: a step of making a semifinished part by press-forming a metal plate so that a pair of half portions of a closed cross-sectional structure extend upwardly from ends of a connecting part having a flat cross section; a step of making the half portions of the closed cross-sectional structure extend further upwardly while forming the connecting part into a bent part having a V-shaped cross section by pressing the connecting part from the inside by using a flat punch inserted into a space between the pair of half portions of the closed cross-sectional structure; and a step of causing outer ends of the half portions of the closed cross-sectional structure to be butted against each other and welding the outer ends after withdrawing the flat punch from the space between the pair of half portions of the closed cross-sectional structure.
Japanese Unexamined Patent Application Publication No. 2006-116552 discloses methods of forming structures having closed cross sections that are circular, rectangular, pentagonal, and polygonal. With that approach, a flat punch having a protrusion at an end thereof is inserted into a space between the pair of half portions of a closed cross-sectional structure, and the half portions of the closed cross-sectional structure is made to extend further upwardly while forming the connecting part into a bent part having a V-shaped cross section by pressing the connecting part from the inside using the flat punch.
Thus, it is necessary to form the bent part having a V-shaped cross section when making the half portions of the closed cross-sectional structure extend upwardly. Because the V-shaped bent part is formed by bending the connecting part into a shape having a comparatively small radius (radius of curvature), a crack may be generated at the V-shaped bent part when a material having a low ductility such as a high-tensile strength steel, is used. Moreover, a crack not visible to the naked eye is likely to be generated and a fracture is likely to occur.
Therefore, the method described in Japanese Unexamined Patent Application Publication No. 2006-116552 has a problem related to formability when the method is used to form a structural part of an automobile such as a front side member. If the end of the V-shaped bent part had a round shape, the half portions of the closed cross-sectional structure would extend upwardly to a smaller degree, and therefore it would become difficult to perform welding in the next step.
Moreover, to form a closed cross-sectional structure having curvatures in three-dimensional directions by using the method described in Japanese Unexamined Patent Application Publication No. 2006-116552, it is necessary to form the three-dimensionally curved shapes in the pair of half portions of the closed cross-sectional structure and to form flange portions at ends of the pair of half portions of the closed cross-sectional structure in the width direction with high precision. Accordingly, that method has a problem related to the production cost.
It could therefore be helpful to provide a method and an apparatus that forms a closed cross-sectional structure and having a three-dimensionally curved shape. By using the method and the apparatus, structures used as structural parts of an automobile or the like can be formed with high precision and can be manufactured at a reduced production cost.